1. Technical Field
The present invention generally relates to image projection. More particularly, the present invention relates to slide projection.
2. Background Art
Slide projectors in the past have been primarily designed for front projection onto a light-reflective screen with the slide projector located a distance from the screen either in the audience area or behind the audience. Front projection, however, can be troublesome and awkward. The projector itself can be a distraction to the audience. The room also needs to be darkened for optimum viewing and a person or object in front of the projector casts a shadow on the screen, making it difficult for a speaker to point to areas of or objects in the image on the screen.
Front slide projection has historically been more popular than rear projection, due to cost considerations stemming from slide projector design. However, rear projection, where the slide projector is behind a screen allowing light to pass through it front-to-back rather than reflecting light, offers significant advantages over front projection in many situations. These advantages include higher image quality, even in partially lit rooms often encountered in business meetings, the projector is out of view of the audience to avoid distraction and a speaker can approach the screen without casting a shadow.
The lenses used in rear projection have relatively short focal lengths, giving a wide-angle image, on the order of 25-50 mm, due to the close proximity to the screen. In contrast, the lenses used in front projection are of medium focal length (on the order of 85-105 mm) or long focal length (on the order of 150-180 mm). Since the majority of slide projectors in the past have been designed primarily for front projection, they include lens ports having a deep lens tunnels for mounting longer focal length lenses, the lens tunnels being on the order of 100-125 mm.
While a deep lens tunnel is optimal for front projection lenses with longer focal lengths, it imposes severe constraints on shorter focal length lenses. Essentially, a deep lens tunnel requires a wide-angle lens to include extra lens elements to "pipe" the light from the slide to the end of the tunnel, where the light rays may finally diverge in a cone. Complicated lens designs, such as reverse telephoto arrangements, are needed to effect this, adding both size and considerable cost to the lens. These types of lenses can cost 5-10 times more than conventional front projection lenses, making rear projection cost-prohibitive. Thus, conventional slide projectors can be used for rear projection using such complex and expensive lenses. However, unless the slides are oriented for rear projection, requiring that slides originally oriented for front projection be flipped 180.degree. on a vertical axis, a mirror outside the projector is needed for inverting the final image. The mirror must be aligned properly with the screen and thus adds to the rear projection effort.
If some way could be found to eliminate the need for complicated rear projection lens designs, rear projection could become competitive in cost with front projection. However, front projection cannot be ignored in the process, since some facilities may only be set up for front projection (i.e., they may not have a rear projection screen). Many speakers have talks that they take on the road with prepared slide trays. Slides for rear projection need to be flipped 180 degrees on a vertical axis in the tray for rear projection in relation to front projection. This could be a cumbersome task for the speaker when some facilities will be set up for front projection only and others for rear projection only.
Thus, a need exists for an easy-to-use slide projector utilizing inexpensive rear projection lenses and also being capable of front projection, without having to flip the slides in the slide tray.